The present invention relates to automotive rearview mirrors that automatically dim in response to glare.
Vehicle operators use interior and exterior rearview mirrors to view scenes behind the vehicle without having to face in a rearward direction and to view areas around the vehicle that would otherwise be blocked by vehicle structures. As such, rearview mirrors are an important source of information to the vehicle operator. Bright lights appearing in a scene behind the vehicle, such as from another vehicle approaching from the rear, may create glare in a rearview mirror that can temporarily visually impair or dazzle the operator. This problem is generally worsened during conditions of low ambient light such as occur at night, when the eyes of the vehicle operator have adjusted to the darkness.
Various solutions have evolved to deal with the problem of glare in automotive rearview mirrors. Initially, a prismatic mirror was provided that could be manually changed between a setting of high reflectivity and a setting of low reflectivity. When the operator experienced glare, the operator could manually change the rearview mirror setting to low reflectivity to reduce the glare. The operator could then manually switch the rearview mirror back to high reflectivity after the glare had subsided. Difficulties with manually controlled mirrors include the glare experienced before the mirror could be switched as well as operator distraction caused by finding and operating the mirror control.
Automatically dimming rearview mirrors were designed to eliminate the need for the operator to manually switch the mirror. The earliest designs used a single glare sensor facing rearward to detect the level of light striking the mirror. This design proved to be inadequate since the threshold perceived by the operator for dimming the mirror, known as the glare threshold, varied as a function of the ambient light level. An improvement included a second light sensor for detecting the ambient light level. The glare threshold in these systems is based on the amount of ambient light detected. Among the dual sensor designs proposed include those described in U.S. Pat. Nos. 3,601,614 to Platzer; 3,746,430 to Brean et al.; 4,580,875 to Bechtel et al.; 4,793,690 to Gahan et al.; 4,886,960 to Molyneux et al.; 4,917,477 to Bechtel et al.; 5,204,778 to Bechtel; 5,451,822 to Bechtel et al.; and 5,715,093 to Schierbeek et al.; each of which is hereby incorporated by reference.
Another related approach is described in U.S. Pat. No. 5,760,962 to Schofield et al., which is hereby incorporated by reference, in which an imaging array is used to gather light from behind and beside the vehicle. Ambient light is detected by examining pixels generally looking sideways. The cost of the imaging array, the required lens, and complicated signal processing logic make the use of an imaging array prohibitively expensive for many automotive applications. Also, light collected from a side view less accurately represents the ambient light experienced by the vehicle operator than does light from a forward view.
Improvements in glare reduction additionally occurred when prismatic mirrors having two states were replaced with mirror assemblies including dimming elements capable of providing many levels of reflectivity reduction. The most commercially successful type of variable dimming element is the electrochromic dimmer. Electrochromic devices produce various levels of opaqueness in response to an applied voltage. When placed in front of a reflective surface, an electrochromic device provides a variably reflective surface, the degree of reflectivity based on a control voltage. Typical electrochromic mirrors are described in U.S. Pat. Nos. 4,902,108 to Byker and 5,724,187 to Varaprasad et al. as well as in commonly assigned U.S. patent applications Ser. No. 08/616,967 titled xe2x80x9cImproved EC Layer And Devices Comprising Samexe2x80x9d to Tonar et al., filed Mar. 15, 1996; and U.S. patent application Ser. No. 08/832,596 titled xe2x80x9cAn Improved Electrochromic Medium Capable of Producing A Pre-Selected Colorxe2x80x9d to Baumann et al., filed Apr. 2, 1997, each of which is hereby incorporated by reference.
A key element in the design of an automatic dimming mirror is the type of light transducer used to implement ambient light and glare detection. A primary characteristic of interest in selecting a light transducer type is the dynamic range. The ratio between the intensity of bright sunlight and moonlight is roughly 1,000,000:1, indicating the wide range that must be sensed by the ambient light sensor. Both the ambient light and the glare light sensors must operate within the ranges of temperature, humidity, shock, and vibration experienced within a vehicle passenger compartment. If a sensor is to be mounted in an outside mirror, even harsher operating conditions can be expected. Sensors and support electronics must also be inexpensive to allow the cost of an automatically dimmed mirror to fall within the range deemed acceptable by an automobile purchaser. Transducers should have good noise immunity or be compatible with noise compensation electronics within the sensor for sensitivity at low light levels. Transducers should further have a spectral response similar to the frequency response of the human eye. As a final desirable characteristic, the sensor must be easily integratable into the types of digital control systems commonly found in automotive applications.
One type of light transducer is the cadmium sulfide (CdS) cell. CdS cells are photosensitive resistors exhibiting increasing conductance with increasing light levels. CdS cells have the advantages of being low in cost, having good sensitivity to low light levels, and having a spectral response somewhat similar to that of the human eye. Disadvantages with CdS cells include a high degree of variance between cells, slow response at low light levels, poor environmental stability, and difficulty being assembled by automated electronic manufacturing equipment. CdS cells for sensing ambient light and glare may be incorporated into a full or partial bridge to increase dynamic range. However, the bridge output represents a fixed relationship between ambient light and glare that is often not appropriate throughout the range of ambient light levels.
Another type of light transducer used in automatic dimming mirrors is the discrete photodiode configured as a light-dependent current source. Photodiodes have less variance between parts, better environmental stability, and are more easily adapted to automated manufacturing than are CdS cells. However, photodiodes tend to be expensive and produce very low currents at low light levels. These low currents require special amplification techniques to achieve a useful signal, increasing the cost of the automatic mirror.
A relatively new type of light sensor incorporates a silicon-based light transducer and conditioning electronics on a single substrate. The light transducer generates charge at a rate proportional to the amount of incident light. This light-induced charge is collected over an integration period. The resulting potential indicates the level of light to which the sensor is exposed over the integration period. Light sensors with integral charge collection have many advantages. By varying the integration time, the sensor dynamic range is greatly extended. Also, the ability to incorporate additional electronics on the same substrate as the transducer increases noise immunity and permits the sensor output to be formatted for use by a digital circuit. Component integration additionally reduces the system cost. Silicon devices are more temperature invariant than CdS cells and can be packaged to provide the necessary protection from humidity, shock, and vibration. One disadvantage of silicon-based light transducers is a frequency response different than that of the human eye. Types of charge accumulating light transducers include photodiodes and photogate transistors. A variety of charge integrating photodiode devices have been described including those in U.S. Pat. Nos. 4,916,307 to Nishibe et al.; 5,214,274 to Yang; 5,243,215 to Enomoto et al.; 5,338,691 to Enomoto et al.; and 5,789,737 to Street. Photogate transistor devices are described in U.S. Pat. Nos. 5,386,128 to Fossum et al. and 5,471,515 to Fossum et al. Each of these patents is hereby incorporated by reference.
One difficulty with all types of light sensors is the occurrence of operating anomalies at high temperatures. Some devices become extremely non-linear at high temperatures. Some devices, such as CdS cells, may suffer a permanent change in operating characteristics. Devices may even provide completely false readings such as indicating bright light in low light conditions due to excessive thermal noise. Traditionally, the only way to deal with this problem has been to incorporate a temperature sensor and associated electronics into the automatic dimming mirror.
What is needed is an automatic dimming mirror that derives the benefits provided by semiconductor light sensors with integral charge collection. The mirror should operate over a wide range of lighting conditions and be less susceptible to temperature variations.
It is an object of the present invention to provide an automatically dimming mirror capable of operating over a wide range of lighting conditions.
Another object of the present invention is to provide an automatically dimming mirror utilizing charge integrating semiconductor light sensors.
Still another object of the present invention is to provide an automatically dimming mirror with less susceptibility to temperature variations.
Yet another object of the present invention is to provide an automatically dimming mirror that is inexpensive to produce.
A further object of the present invention is to provide an automotive rearview mirror system with improved exterior mirror performance.
In carrying out the above objects and other objects and features of the present invention, a rearview mirror for an automotive vehicle allowing a vehicle operator to view a scene generally behind the vehicle operator is provided. The mirror includes a dimming element having a variably reflective surface. The degree of reflectivity is based on signals from an ambient light sensor positioned to receive light from a region generally in front of the vehicle and from a glare sensor positioned to view the scene generally behind the vehicle operator. At least one of the ambient light sensor and the glare sensor accumulates charge in response to incident light over a variable integration period.
In another rearview mirror, the ambient light sensor outputs a discrete ambient light signal based on the amount of light incident on the ambient light sensor over an ambient light integration period. The glare sensor outputs a discrete glare signal based on the amount of light incident on the glare sensor over a glare integration period. Dimming logic determines an ambient light level based on the ambient light signal. The glare signal resulting from the glare integration period determines a mirror glare level used to set the dimming element control signal.
In an embodiment of the present invention, the ambient light sensor has an input for receiving an ambient light integration period signal specifying the ambient light integration period. The dimming logic outputs the ambient light integration period signal based on at least one previously determined ambient light level.
In another embodiment of the present invention, the dimming logic generates a sequence of ambient light integration period signals, each integration period signal specifying a different light integration period. The ambient light level is determined based on a resulting ambient light signal having a pulse width within at least one preset width threshold.
In still another embodiment of the present invention, at least one of the ambient light sensor and the glare sensor is a light sensor including an exposed light transducer which accumulates charge in proportion to light incident over an integration period. Sensor logic determines the light integration period prior to beginning integration. Charge accumulated in the exposed light transducer is reset at the beginning of the light integration period. Charge accumulated by the exposed light transducer over the light integration period is measured and a pulse having a width based on the measured accumulated charge is output.
In yet another embodiment of the present invention, the light transducer further includes a light transducer shielded from ambient light. The shielded light transducer accumulates charge in proportion to noise over the integration period. Sensor logic resets the charge accumulated in the shielded light transducer at the beginning of the light integration period. The charge accumulated by the shielded light transducer over the integration period is measured and a pulse having a width based on the difference between the measured accumulated exposed light transducer charge and the measured accumulated shielded light transducer charge is output.
In a further embodiment of the present invention, the light sensor receives an integration pulse having a width determining the integration period. The output pulse is generated after the integration pulse is received. The difference in time between the end of the integration pulse and the start of the output pulse indicates the amount of thermal noise in the light sensor. In a refinement, the rearview mirror dimming logic determines the amount of time between the end of the integration pulse and the start of the output pulse. The light sensor temperature is determined based on the elapsed time. The dimming logic may disable automatic dimming of the dimming element reflective surface if the light sensor temperature exceeds a preset limit.
In a still further embodiment of the present invention, at least one of the ambient light sensor and the glare sensor is a light transducer including an enclosure having a window for receiving ambient light. The enclosure admits a power pin, a ground pin, and a signal pin. An exposed light transducer within the enclosure accumulates charge in proportion to light received through the window and incident on the exposed light transducer. A light-to-pulse circuit within the enclosure outputs a pulse having a pulse width based on charge accumulated by the exposed light transducer over an integration period. Sensor logic within the enclosure receives an integration pulse on the signal pin, determines the integration period based on the width of the integration pulse, and outputs the output pulse on the signal pin.
In yet a still further embodiment of the present invention, the dimming logic includes at least one signal pin, each signal pin connected to the signal pin of the ambient light sensor or the glare sensor. For each dimming logic signal pin, the dimming logic sets the signal pin to output mode. An integration period is determined and an integration pulse generated on the signal pin. The signal pin is set to input mode. The light sensor output pulse is received and a light level determined based on the light sensor output pulse duration.
In an additional embodiment of the present invention, a glare lens is provided for the glare sensor. The glare lens provides the glare sensor with a narrower field of view and a higher optical gain than the ambient light sensor.
A rearview mirror system for an automotive vehicle is also provided. The rearview mirror system includes an interior rearview mirror and at least one exterior rearview mirror. In one embodiment, each mirror contains a dimming element, an ambient light sensor, a glare sensor, and a dimming logic. The dimming logic determines the glare integration period based on the ambient light level and uses the resulting glare level to determine the dimming element control signal. In another embodiment, each exterior mirror contains a glare sensor but no ambient light sensor. Each exterior mirror determines the glare integration period based on an ambient light signal from the interior rearview mirror. Each exterior mirror determines a dimming control value based on glare level obtained from the exterior mirror glare sensor. In still another embodiment, exterior mirrors contain no light sensors and receive a signal indicating the degree of reflectivity for exterior dimming elements from the interior rearview mirror.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.